fix: Front-end React deprecations

[camd]

Not complete yet, and not exhaustive. But this fixes a LOT of them.
It's large, so I tried to break types of fixes into different commits. But I wasn't perfect in that regard. I started getting more careful in the later commits. 😆

A few details:

• I build it on top of the react-bootstrap PR, as it is a natural continuation.
• I added new Puppeteer tests and fixed up the existing ones. I will ensure they do not run in CI by default, though.
• I also fixed up many of the warnings and errors in the React Testing Library tests.
• Upgraded the hot-reloading package, which was causing warnings as well.
• Upgraded the "Helmet" package which handles headers and metadata, etc.

TODO:

• Have CI run the puppeteer tests by default
• Check how slow it is
• Does it increase coverage

[Archaeopteryx]

Please rebase here when you find time.